The present invention relates to following tracking helical data tracks with rotating tape access heads.
Helical scan tape systems record tracks on magnetic tape at an angle with respect to the edge of the tape by means of a rotary, or helical, tape head. The helical scan system produces high density recording by writing data tracks at an angle across the width of the tape, resulting in high-speed tape access by read and write elements on the tape head without the need for equally high-speed tape motion. Typically, a pair of data tracks, also known as data channels, are simultaneously written onto or read from the tape. Write elements on the tape head are angled in opposing directions so that bit patterns on adjacent track pairs are similarly angled. Typically, there are two pairs of read elements and two pairs of write elements on the tape head, with alternating pairs contacting adjacent data tracks as the tape head rotates.
Data tracks contain data to be stored on the tape together with additional information such as error correction and detection bits and synchronization patterns. When data is read back from the tape, the error correction and detection bits are used to detect and correct data errors that may occur due to debris on the tape, mechanical damage of the tape, tape head tracking errors, and the like. However, only a certain number of erroneous bits may be corrected or detected within the span of data protected by the error correction and detection bits.
Synchronization patterns are used to align the tape head with the data channel pairs, reducing head tracking errors. These patterns are typically written at regular intervals along the length of each data track. Ideally, if the tape head drifts relative to the data channel pair, a synchronization pattern for one track will be read by the read element for that track at a different time than the corresponding synchronization pattern on the second track will be read by the read element for the second track. Control logic uses this time difference to move the head so that read elements are better positioned to read the data tracks.
There are several problems with this head positioning system. The control logic assumes that write element pairs and read element pairs are located on the tape head so as to access a data channel pair at the same location along the length of each data channel. Due to tolerances in the manufacturing process, for example, this may not be the case. Any offset between elements in an element pair will be interpreted by the control logic as a head tracking error. This problem is compounded by the possibility for offset in both read element pair location and write element pair location. A further complication is the variance introduced by using different tape systems to write the data and subsequently read the data. Other factors, including tape wear, tape stretching, temperature effects, and the like, may introduce still further sources of disturbance in calculating head tracking position.
What is needed is a helical tape tracking system and method that compensates for variations in read and write element positioning relative to a data track. Improved track following should be attained without significantly affecting the operation or performance of the tape system or adding significant cost. A system including the improved track following features should operate with tape recorded on previous tape systems and should produce tapes that can be read by previous tape systems.
Accordingly, the present invention should detect and compensate for variations in the position of read and write elements accessing a given pair of data channels. This may be accomplished by using the relative offset of the first detected synchronization patterns in each pair of data channels as a measure of the relative locations of write elements producing the synchronization patterns and read elements accessing the synchronization patterns. This relative offset is then used to correct subsequent synchronization pattern readings taken from the data channel pair.
A helical scan tape drive is provided that improves track following. The drive includes a scanning tape head with at least one read element pair. Each read element pair has a first read element and a second read element not parallel with the first read element. The read elements concurrently read a first helical data track and a second helical data track written onto magnetic tape, each data track having a plurality of spaced apart synchronization patterns. A tape drive moves magnetic tape having a plurality of helical data tracks past the tape head. A servo positions read elements across the data track pair. A control unit detects synchronization patterns read from the first data track and the second data track. A first time interval between synchronization patterns detected from the first data track and the second data track is determined at the start of the first data track and the second data track. Additional time intervals between synchronization patterns detected from the first data track and the second data track are determined after the start of the data tracks. A tape head track offset is determined based on the first time interval and at least one additional time interval. The control unit may thus determine relative position between the write elements that wrote synchronization patterns appearing on the data track pairs, between read elements reading the synchronization patterns, or the combined effect of both.
In an embodiment of the present invention, the control unit includes a clock generating clock pulses. A counter counts clock pulses between synchronization patterns detected from the first data track and the second data track. A memory holds the counter value for the first time interval. Logic determines the difference between the counter value held in the memory and the current counter value.
A method of compensating for data track path variance seen by a tape head accessing a pair of helical data tracks recorded on magnetic tape is also provided. A first time is determined between detecting a first synchronization pattern on each of the helical data tracks at the start of reading the pair of data tracks. At least one subsequent time is determined between detecting a subsequent synchronization pattern on each of the helical tracks. The data track path variance is then determined based on the first time and the subsequent time. A control signal may then be generated to move the tape head relative to the data tracks.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.